Their size may vary between a few nanometers for the droplets present in clouds, and a few tens of microns for the dust particles generated by human activities or by natural effects.
The effects of these particles on human health are becoming an increasing concern.
They depend essentially on the capacity of these particles to be absorbed by the respiratory tracts.
The particles which have a large size remain trapped in the nasopharyngeal cavity, while the particles with a smaller size are capable of penetrating into the alveolar section of the lungs.
Likewise, the fraction of particles exhaled gradually increases with the dimension of these particles.
Consequently, the particles which have the smallest size are considered to be those most dangerous for human health.
This is why monitoring the level of human exposure to the particles is an important element in evaluation of the health risks at the place of work and in the outside environment.
This monitoring requires simultaneous measurement of the concentration and the size of the particles present in the ambient air for at least one given particle size.
Thus, the national and European standards currently in force require monitoring of the PM10 parameter, that is to say the number of particles per unit volume which are present in the atmosphere and have a dimension greater than 10 μm. Other parameters may also be envisioned, such as PM2.5 or PM1, that is to say the number of particles per unit volume having a dimension greater than 2.5 μm or 1 μm.
In the prior art, there are three known techniques used for measuring the concentration of particles in the atmosphere: gravimetric measurement, the β technique and the technique of optical absorption and diffraction.
Gravimetric measurement consists in filtering the ambient air with the aid of a gravimetric filter having controlled porosity. This filter collects all the particles below a given size, referred to as the cutoff size.
It is generally associated with a second filter having selective admission, which removes the particles which have a large size, and with a pump which ensures a constant air flow throughout the system.
In order to determine the quantity of particles, the filter is subsequently weighed.
Thus, in order to obtain information about the size of the particles, it is expedient to use a plurality of different filters.
This technique requires the use of numerous consumables and the conduct of a large number of operations. This is why it is relatively time-consuming and expensive to implement.
Lastly, this technique only makes it possible to deliver an average measurement and cannot provide any information about the distribution of the particles as a function of time.
The β technique uses a low-energy carbon-14 source which provides a constant flux of β electrons that are detected by a Geiger tube or by a matrix of photodiodes.
A band filter is interposed between the source and the detector.
The particle measurement cycle starts with calibration.
A gas sample is subsequently sent through the band filter, on which all the particles whose size is greater than a given size, for example 10 μm, are collected.
The filter is subsequently interposed again between the source and the detector, and the transmission of the β electrons is measured.
The difference in β electron transmission through the filter is directly proportional to the mass of particles accumulated on the filter.
Like the gravimetric technique, this β technique uses a fairly large quantity of consumables. Furthermore, it only makes it possible to provide an average measurement, and it therefore does not provide any information about the distribution of the particles as a function of time.
It may also be noted that information about the size of the particles requires the use of a plurality of filters.
Lastly, the β technique requires the presence of a radiation source and a system capable of managing complex operations.
The technique of optical absorption and diffraction is based on measurement of the amount of light diffracted by the particles present in the working volume of a detector.
Mention may thus be made of a photometric detector which measures the amount of light scattered by the interaction with the particles.
Such a detector makes it possible to cover a fairly wide range of particle concentrations.
However, the signal which it delivers is proportional to the size, the shape and the optical properties of the particles, and cannot provide an estimate of the average size of the particles.
Furthermore, the estimate of the concentration depends on the difference between the amount of light absorbed and the amount of light transmitted. For this reason, the difference becomes very small when the particle concentration decreases and it is no longer possible to make a correct estimate.
Document DE-4230087 describes another type of particle detector which comprises a substrate, with an etched groove and an integrated waveguide, as well as a light source and a light receiver which are independent of the substrate. The waveguide is interrupted by the etched groove which allows passage of the medium to be analyzed. Furthermore, a membrane provided with holes is provided in order to determine the minimum size of the particles to be measured, and in order to filter the flowing medium.
Thus, with this detector the particles are prefiltered and their passage between the two parts of the waveguide subsequently leads to dissipation of the light. The difference in light flux passing through the two parts is detected and related to the number of particles passing through the groove.
Such a detector is relatively complex, because it requires prefiltering. Furthermore, the use of discrete components conventionally entails adjustment problems.
Mention may also be made of the optical particle counter, which uses a laser source focused in proximity to an air jet containing the particles to be detected. The lighted diffracted, scattered or reflected by the particles is collected by a photodiode, which is off-center with respect to the optical axis.
Such an optical particle counter is described in particular in the article “A novel optical instrument for estimating size segregated aerosol mass concentration in real time” by X. Wang et al. (Aerosol science and technology, 1 Sep. 2009).
An optical particle counter has the benefit of delivering the number of particles and an estimate of their size in real time, owing to the measurement of the intensity of the light collected.
However, the quality of the measurements is contingent on precise positioning of the laser beam and of the air jet.
One major drawback of such an optical counter is the risk of underestimating the concentration of particles. This may be due to the simultaneous presence of a plurality of particles in the detection volume, these particles being partially or completely superimposed in relation to the laser beam.
Furthermore, an optical counter conventionally consists of discrete components, which raises problems of bulk, alignment and adjustment.
Thus, document WO97/12 223 describes a flow cytometer comprising two components: an optical head and a disposable flow module. The optical head comprises a laser and two photodetectors. The flow module consists of a substrate etched with a channel for the passage of a flow.
This document indicates that the optical components may be mounted in a rigid housing in order to preserve their alignment. However, these are discrete components which are furthermore independent of the module, since the latter is a disposable element.
In all cases, devices consisting of discrete elements also lead to significant manufacturing costs.